1. Field of the Invention
This invention relates generally to a technique for cooling a power transfer unit by flowing coolant over surface where heat can be transferred to the coolant and away from the unit.
2. Description of the Prior Art
A power transfer unit (PTU) is a mechanical device that transmits power from the output of a transmission to a driveshaft leading to the real vehicle wheels. In a vehicle whose front wheels are continually driven by the transmission, the PTU changes the direction of rotating power flow from a lateral direction to a longitudinal direction.
A recent trend in vehicle powertrain design provides all wheel drive in more vehicles and delivers more power to the wheels of a secondary axle.
The PTU is subject to temperature increase in service. Friction developed in the PTU produces internal heat. Because automotive components that operate at high temperatures, such an engine, turbocharger, engine exhaust pipe and catalytic converter, are located near each PTU in the engine compartment, the air surrounding the PTU is at high temperature, which limits potential heat transfer from the PTU to ambient air.
Conventionally no cooling system is provided to carry heat from a PTU. Therefore, heat is rejected from a PTU primarily by conduction to the transmission and convection to the surrounding air stream. But underbody shields limit heat transfer potential by reducing air movement over the PTU.
Due to these factors and conditions, the temperature of the PTU and its case are reaching levels where the functional of the PTU can be compromised or adversely affected.
As a result of transmitting more rotating power through the PTU, more heat rejection from the PTU is required to maintain its temperature at an acceptable magnitude. Consequently, a need exists in the industry for a system that cools a PTU, preferably by continually circulating coolant over surfaces of the PTU so that heat can be transferred to the coolant and away from the PTU.